Fixed #24733 -- Passed the triggering exception to 40x error handlers

Thanks Tim Graham for the review.

docs/releases/1.9.txt

@@ -269,6 +269,9 @@ Requests and Responses
 
 * The debug view now shows details of chained exceptions on Python 3.
 
+* The default 40x error views now accept a second positional parameter, the
+  exception that triggered the view.
+
 Tests
 ^^^^^
 
@@ -520,6 +523,10 @@ Miscellaneous
   Therefore, the ``@skipIfCustomUser`` decorator is no longer needed to
   decorate tests in ``django.contrib.auth``.
 
+* If you customized some :ref:`error handlers <error-views>`, the view
+  signatures with only one request parameter are deprecated. The views should
+  now also accept a second ``exception`` positional parameter.
+
 .. removed-features-1.9:
 
 Features removed in 1.9